Hi Richard,
The next steps I would try is grant read permission on the mojoportal root folder to the "Network Service" user and then grant the same user full control on the /Data folder.
How are you resolving the hostname "mojo"? Can you try running it just as http://localhost?
Usually on Vista you only have 1 root web site, the Default Web Site which runs at http://localhost, and you either run a .NET app at the root of that site as http://localhost or you run it as virtual directory application and the url is http://localhost/appname.
If you have a .NET app running in the root web site and .NET app(s) running as virtual directories there can be conflicts between web.config files. Usually its the apps in the virtual directories which get broken because the settings in the root app are overriding the web.config in the virtual directory. It can be resolved by pointing the root web site somewhere else like an empty folder where there is no Web.confiog file.
Other things you can try are changing the Application Pool from "Integrated" to "Classic".
Removing the <system.webServer> and <system.web.extensions> and <system.serviceModel> sections completely from the mojoportal Web.config (back it up first in case that doesn't solve it).
Hope it helps,
Joe